In conventional color photography, the photographic products contain three superimposed units of layers of silver halide emulsions, one to form a latent image corresponding to exposure to blue light (blue sensitive), one to form a latent image corresponding to exposure to green and one to form a latent image corresponding to exposure to red.
During the photographic processing, the developing agent reduces the silver ions of each latent image. The developing agent oxidized during this reaction then reacts in each unit with a dye-forming coupler in order to produce yellow, magenta and cyan dye images respectively from the recordings in blue, green and red. This produces negative dye images.
The reversal photographic products which give positive images comprise the three same superimposed units of layers of silver halide emulsion, each of these units containing respectively a yellow, magenta and cyan dye-forming coupler. After exposure, these reversal photographic products are subjected to a first black and white development (development of the latent image) and then to a reversal step which is either chemical or through a further fogging exposure which makes developable the silver halides which were not initially exposed. After reversal, the photographic product is processed in a color development bath in the presence of couplers, generally contained in the photographic product.
In the art it is known that silver halide photographic films can be used for making advertising media (posters) which can be used as displays. These films, after having been exposed and developed are exposed on high-intensity light, the image being continuously illuminated. For example, the photographic product DURATRANS.RTM. manufactured by Kodak.RTM. requires the use of a negative/positive system of the Ektacolor.RTM. type, that is to say the DURATRANS.RTM. film is a film for a light box which requires the use of a negative original image. For obtaining a film for displays from a transparency, it is necessary to perform an intermediate trial on an intermediate negative film in order to obtain the final positive print for the light box. In addition, this system has the drawback of not being totally compatible with the standard sequence of the Ektacolor.RTM. processing method currently used in processing laboratories. In particular, the duration of each processing step must be modified with respect to the standard durations, which makes its use unsuited to automated processing.
There also exists a film for displays intended for a method of processing by local destruction of dyes in the presence of a silver image known as "silver dye bleach". This method is used with photographic film on which the layers of silver halide emulsion are initially colored by means of a cyan, magenta and yellow non-diffusing dye. During the development, three superimposed negative silver images are formed, embedded in the mass of three dyes. In order to have a colored image in each layer, it is necessary to destroy the dye, proportionally to the quantity of silver present locally. The duration of the photographic development with such a method is greater than 10 minutes.
Having regard to the existing photographic films for displays, it is desirable to develop a novel photographic film for light boxes which does not present the drawbacks of the known films. In particular, it is desirable to have a novel film for light boxes using reversal technology which is also entirely compatible with conventional processing methods.